Materials in gas turbine engines must have both exceptional elevated temperature mechanical properties and resistance to surface degradation such as oxidation and hot corrosion at elevated temperatures.
In current gas turbines temperatures range in excess of 2500.degree. F., and it is desired that such engines operate for periods in excess of 10,000 hours without undergoing significant materials deterioration. To meet these goals, all of the components in the hot turbine section of such gas turbine engines are coated with protective coatings.
Such coatings are of two general types, aluminide coatings and overlay coatings. Aluminide coatings are produced by diffusion of aluminum into the part to be protected and the reaction of the aluminum with the substrate material to produce intermetallic compounds. In use, the part develops an alumina layer which acts as a barrier to prevent the further oxidation of the coated part. Attempts to use similar silicon based coatings directly on nickel superalloys have been unsuccessful because the nickel-silicon compounds which form have low melting points.
The second major type of coatings are those which are termed overlay coatings. Overlay coatings are themselves oxidation resistant and do not depend upon any reaction with or diffusion into a substrate.
Typical of the more successful overlay coatings are those termed "MCrAlY" coatings where "M" is nickel, cobalt, iron or mixtures thereof. Such MCrAlY coatings are described and claimed in U.S. Pat. Nos. 3,542,530; 3,676,085; 3,754,903 and 3,928,026 which are all assigned to the assignee of the present invention and which are incorporated herein by referencce. Again, such coatings derive there resistance to environmental attack from the formation of an alumina surface layer.
Other types of overlay coatings have been evaluated and have utility in connection with the present invention. These include the MCr type coatings, the MCrAl type coatings and the MCrAlHf coatings (U.S. Pat. No. 3,993,454). In all of these coatings "M" may be iron, nickel, cobalt or mixtures thereof.
A great deal of effort has been expended in the development of coatings which provide environmental stability at extremely high temperatures, for example, 2100.degree. F., and for such high temperature applications, the MCrAlY type coatings are generally unsurpassed.
Some recent work has uncovered an unexpected form of corrosion which occurs at lower temperatures of from about 1200.degree. to about 1400.degree. F. and which reaches a peak in the 1300 .degree. to 1350.degree. F. range.
The basic concepts of pack cementation coatings, which are also referred to as diffusion coatings, are presented in an article by R. L. Wachtell found in "SCIENCE AND TECHNOLOGY OF SURFACE COATINGS," published by Academic Press, New York., 1974, pgs. 105 to 119.
An article entitled "STRUCTURE AND PROPERTIES OF SILICIDE BASED DIFFUSION COATINGS" by H. van Amerongen describes certain silicon rich coatings and is found in the book entitled "HIGH TEMPERATURE ALLOYS FOR GAS TURBINES," published by Applied Science Publishers, London, 1978, pgs. 209 to 224.
A further reference to silicon based coatings is found in the preceedings of the "THIRD INTERNATIONAL CONFERENCE ON CHEMICAL VAPOR DEPOSITION," Salt Lake City, Utah, April 1972, by P. C. Felix et al. entitled "CVD SILICON COATINGS FOR THE CORROSION PROTECTION OF TURBINE BLADES".
U.S. Pat. Nos. 3,873,347 and 3,874,901 describe coating systems in which aluminide coatings are applied to articles which have previously been coated with an MCrAlY coating.